add concat resolver for files with factory

RWOverdijk/AssetManager#110
add tests for ConcatResolver

RWOverdijk/AssetManager#110
set constructor variable for content optional
append content method to concat string asset
add php doc
make concat resolver mime resolver aware
fetch mime type and throw exception if mime type changes in concatenated files
removed unused code
create concat string asset instance  and add content in loop
removed appending content to variable in loop
apply filters to assets
set mimetype in loop
set target path of resolved concat string asset
update tests
Merge branch 'feature/concat_resolver' into develop