Use `none` in `parsebib--biblatex-inheritances`.

In order to resolve issue
[#231](joostkremers/ebib#231) in Ebib, the formats of
`parsebib--biblatex-inheritances` and `ebib-biblatex-inheritances` need to be
synchronised. Using keywords somehow feels less Elisp-y, and the Ebib version is
user-customisable, where a plain symbol may be more user-friendly.

Update copyright statement.

README: Add section on `parsebib-parse`

Bug fix: Use `parsebib--get-hashid-string` in `parsebib-read-entry`.

Remove support for Emacs 24.

Update version number to 2.4.

Use bibtex-end-of-entry to search for closing parenthesis.

The hack that this commit replaces (searching for the next entry and then moving
back until a closing parenthesis is found) fails when the .bib file contains
material that is not part of any entry. BibTeX allows this, so parsebib should
as well.

Note that bibtex-end-of-entry works by parsing the entire entry, so it's
probably not as efficient. A better solution would be to get rid of the function
parsebib-match-paren-forward and detect the end of an entry through
parsebib--find-bibtex-field.

See also joostkremers#8 for discussion.

Use char-before in parsebib--match-paren-forward.

Searching a closing parenthesis would fail on correctly formatted BibTeX entries
due to the fact that point would end up after the matching parenthesis,
while (char-after) assumes it's before it.

This fixes joostkremers#8.

Allow $ ^ and & in BibTeX keys and identifiers.

Apparently these characters are allowed in keys etc., even though using them is
ill-advised, given the fact that they are special for TeX.

Update version number.

Include general inheritances when using biblatex scheme