Files Service: add createFile() method (#506).

The server-side `FilesService` provides a new method:
{{{
Topic createFile(InputStream in, String path);

@param   in     The input stream the file content is read from.
@param   path   The path and filename of the file to be created.
                A repository path. Relative to the repository base path.
                Must begin with slash, no slash at the end.
                If that file exists already it is overwritten.

@return  the File topic that corresponds to the created file.
}}}

If you have an input stream at hand you can pass it directly.

If you want write a String to the file:
{{{
String str = ...
InputStream in = new ByteArrayInputStream(str.getBytes("UTF-8"))
Topic file = filesService.createFile(in, "/newfile.txt")
}}}

This example would create the file in the root directory of the DeepaMehta file repository.
Remember: by default the DeepaMehta file repository is your entire disk.
Consider setting the <dm4.filerepo.path> config property (in global pom.xml) to a directory that is writable, e.g.:
{{{
<dm4.filerepo.path>/home/alice/deepamehta-files</dm4.filerepo.path>
}}}
The specified directory must exist.

=> The `createFile()` method is not yet tested. Please test it!

See #506.