Added default context configurations

This release includes mainly a feature that allows to configure the default context options. The feature has been implemented mainly by @edefimov , Thanks!

Example:

jms_serializer:
  default_context:
    serialization:
      serialize_null: false
      version: ~ 
      attributes: {}
      groups: ['Default']
    deserialization:
      serialize_null: false
      version: ~ 
      attributes: {}
      groups: ['Default']

Implemented enhancements:

• Added configuration for default context #556 (edefimov)

Merged pull requests:

• Use svg build badge #560 (hanneskaeufler)

Closed issues:

• Document how to prevent JMS serializer from overriding Symfony serializer #513
• AccessorOrder : properties vs virtualProperty #512
• Specify @Serializer/Group for @Discriminator field #506
• Ignore entity and/or subentity based on attribute value #499
• How to change Strategy #493
• AccessorOrder is ignored #488
• When serializing subentities discriminator value is always added to serialized output #479
• Serialized name "id" #461
• Jms serializer @jms\Inline() annotation overrides an actual id #460
• @jms\Serializer\Annotation\Type("DateTime<'c'>") doesn't works properly #459
• Custom Class Type Mapping #446
• Return an array instead of object #439
• Serializer does not seem to visit virtual properties recursively #429
• filtering doctrine entites with ExclusionPolicy\(All\) #419
• [Feature] Apply exclusión policity by groups #401
• DateTime ISO8601 in PHP doesnt support milliseconds #395
• custom handler documentation #379
• Question: How to serialize data for KNP paginator which is using Solarium Subscriber. #374
• Error View::create #365
• Propel Collection Handler is not registered #349
• Cannot redeclare class doctrine\orm\mapping\annotation #339
• get a error of JMSSerializerBundle in Symfony2.3.4 and ODM #320
• DoctrineObjectConstructor doesn't work with id as attribute #305
• Performance issue - Help needed for optimization #281
• GraphNavigator - Marked as Final #238
• Error when using yml for FormErrors #221
• Can't configurate class which is in global namespace with YAML or XML #217
• VirtualProperties called after postSerialize triggers #216
• What's the purpose of 'id' under 'property_naming' namespace in configuration. #522
• "fos_rest.serializer" must implement FOS\RestBundle\Serializer\Serializer (instance of "JMS\Serializer\Serializer" given). #509
• property_naming.id is not used? Property way to override the naming strategy #449
• add option to alias field names #433