Added GraphQLOperation annotation support.

[jeremygiberson]

References #16

• set maven default build source/target to 1.7. (should be set to min compile level at least, 1.7 felt safe)
• add annotation, tweaked argument annotation to add variable reference
• refactor requestentity wrapping top level property w/ operation
• added test scenario for working with operations.
• todo - more tests to exercise each operation parameter

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[jeremygiberson]

Btw, do you have recommendation on how to run style linting locally so I don't have to push & wait for travis ci to check fixes?

[chemdrew]

Hey @jeremygiberson, thanks for opening the PR! I really like the idea of adding the operation, this makes it a lot clearer on what the root node of the property tree we are building is instead of checking for null fields. To run the tests with checkstyle + coverage checks I recommend mvn clean verify