Automatically detect class path for Java (Eclipse)

Relevant to #417.

I'm using ALE with eclim and the default Eclipse project structure (Java source files directly in `src/`). By default ALE won't find my .java files.

For now I have solved it by changing [line 43 in ale_linters/java/javac.vim](https://github.com/w0rp/ale/blob/a43ada93e40b8286dde3cd62f10369876787ddc1/ale_linters/java/javac.vim#L43) to:
`let l:src_dir = ale#path#FindNearestDirectory(a:buffer, 'src/')`

This probably won't play well with the Maven project structure though. Eclipse uses an xml file (`.classpath`) in the root of the project to store the paths, maybe that can be used.