Summary: added various options and fixed a possible bug in paper IDs

* allow upper-case characters in bibtex keys

* fixed a bug (?) with possible whitespace in paper id

* added an option to change semantics of ALL in bibfilter
Originally, bibfilter restricts the entries to authors and types given
in the select boxes. This means, ALL will not show all available
papers, but all selectable papers.
I have added options to change the semantics of ALL to mean "all
available" papers. This allows to specify only a few authors or types
in the select boxes (for example, choose between journal paper,
conference paper, or ALL), but selecting ALL will then display also
techreports and manuals.

* added an option to select bibtex file in bibfilter
By default, bibfilter will use all the files given with the 'file'
option (i.e., it will use all files at once). Rather than using them
all at once, we may want to give the user the choice which file to
use, for example to select different bibliographies. This new option
adds a new select box to choose files, to achieve exactly that.