Handle json null values

Merge pull request #54 from BlackEdder/null

Fix #53: JSON_TYPE.NULL results in exception

Support dmd 2.071.0

v1.3.7

Make field mutable (being assigned in constructor)

Support latest dmd compiler

Merge pull request #50 from BlackEdder/imports

Support new compiler

Further improvements to associative array handling

Hopefully fully fix #44

Improved associate array hanling

Thanks to @Herringway for reporting this and @Zalastax for fixing associative array handling.

Fix import in release version

Newer versions of dmd are stricter (more correct) with respect to imports. This fixes the imports for the release version (test version was fine)

Relicense to boost

Following discussion in #40 we decided to relicense painlessjson with the Boost license. This will bring painlessJSON more in line with other D projects/libraries and allow users to distribute binary code without having to credit us.

Also added additional compilers to the continuous integration testing (Travis)

v1.3.1: Merge pull request #37 from Zalastax/master

Fix import mixin to resolve issue #36