Added functionality to assign field ids manually in a schema

New attribute:

-   `id: n` (on a table field): manually set the field identifier to `n`.
    If you use this attribute, you must use it on ALL fields of this table,
    and the numbers must be a contiguous range from 0 onwards.
    Additionally, since a union type effectively adds two fields, its
    id must be that of the second field (the first field is the type
    field and not explicitly declared in the schema).
    For example, if the last field before the union field had id 6,
    the union field should have id 8, and the unions type field will
    implicitly be 7.
    IDs allow the fields to be placed in any order in the schema.
    When a new field is added to the schema is must use the next available ID.

Change-Id: I8690f105f3a2d31fdcb75a4fab4130692b12c62f
Tested: on Windows
Added a "strict JSON" mode to the text generator and compiler

This will add quotes around field names, as required by the official
standard. By default it will leave quotes out, as it is more readable,
more compact, and is accepted by almost all JSON parsers.
The -S switch to flatc turns on strict mode.

As per rfc 7159.

Change-Id: Ibabe9c8162c47339d00ec581d18721a2ba40c6d0
Tested: on Windows.
Fixed vectors reserving wrong amount of space in Java.

In the generated code, the type of the vector was used for
the element size, instead of the vector element type.

Change-Id: Ie16d5221a61474365292c948fd3af99e2a7716c7
Tested: on Windows.
Fixed incorrect verifier code for nested tables.

It was outputting the type instead of the field name, and didn't deal
with NULL fields. Added test case.

Also fixed token enums having the wrong value, resulting in
unreadable error messages.

Change-Id: Icd9b4d22f417bfad5824c0f58e067ce3f2e2dc6f
Tested: on Windows and Linux.
Fixed namespace declaration not being cleared between files.

Change-Id: Ie9fedf894d82a403d0b6b4848d221d6bbee58984
Tested: on OS X.