Added first attempt to implement generated enum type for @JsonEnumProperty

Eventually it must be able to reuse existing enum types as well as generate them.
added half a working sample
Added @JsonEnumProperty, that either generates an enum for you or uses an existing enum type,
to map String values to the corresponding value of the enum type
After some deliberation, I decided that a @EnumProperty should exist
independently from @JsonProperty.

After all it's all about converting Strings to enum type values.

Conflicts:
	Xtendroid/src/org/xtendroid/json/JsonProperty.xtend
Made methods static, so they can be imported as static extensions.
Changed the method names to prevent collisions when multiple
enums are used as static extensions.
Removed usused local variable
I should really wait before doing a merge request... more clean up.
This should be the last one.
Merge pull request #26 from Buggaboo/merge_upstream_enum_property

Merge upstream enum property