filter.[ch]: new function filter_expression_info()

- new function filter_expression_info() to print out
  the filter expression help message. could be called
  tools that use filter.h
.gitignore: ignore *.so files
add Program info line to main bcftools help message to match samtools
more updates to vcfview options

* replace `--biallelic`/`--multialleic` options with more general `--min-alleles`/`--max-alleles`

* drop the `--exclude-ref` option as it is covered by `-c1` option; test updated to reflect this

* optionally specify that `--min-ac` and/or `--max-ac` are referring to the non-ref alleles (default), minor allele or first alternate allele

* add options `--min-af` and `--max-af` similar to `--min-ac` and `--max-ac`

* fix typo in `--known`/`--novel` description

* long option `--include-types` changed to `--types` for consistency

* new option `--uncalled` to complement `--exclude-uncalled`
resolve merge conflict for vcfview.c
switch allele frequecy short options from -d/D to -q/Q; update docs for view options
add a bunch of tests and bugfixes for new view options
Merge pull request #23 from sm15/feature/subset_options

further updates to bcftools view options