add dsp tools
Add mt_pgram (multitaper periodogram)
fix welch_pgram doc
Merge pull request #82 from JuliaDSP/gfa/welch

fix welch_pgram doc
add to util doc
Merge pull request #80 from JuliaDSP/sjk/mt

Add mt_pgram (multitaper periodogram)
Small doc cleanup
Merge pull request #76 from gummif/utils

Added dB conversion and RMS calculation functions to utils