Fix default mode in documentation

Here's the actual default mode: https://github.com/tmm1/stackprof/blob/master/lib/stackprof/middleware.rb#L10
Merge pull request #10 from pedrogimenez/fix-documentation

Fix default mode in documentation
Fix #11
Merge pull request #13 from pedrogimenez/fix-options

Fix #11
warn on invalid files
save combined profiles with --dump
Allow for passing a specific filename to Middleware.save

Also return the filename instead of the return value of .open()
This allows for overwritting the same file when calling .save
manually as well as keeping track of the exact file written (using
the return value)
Merge pull request #14 from quirkey/save_filename

Allow for passing a specific filename to Middleware.save
Add link to stackprof-webnav
Merge pull request #15 from alisnic/master

Add link to stackprof-webnav
Merge branch 'master' of https://github.com/tmm1/stackprof
callers helper
root frames
modeline
add raw sampling option
add out option to export profile to file directly
stackcollapse reporting for flamegraphs
import git://github.com/brendangregg/FlameGraph @ 250bd356837aa62188b3205918b6b05c04d54c93
import https://code.google.com/p/jrfonseca.gprof2dot/ @ 3c962f82e5640941dfb9cbc1abe8dfd3ae2d14da
add stackprof-flamegraph and stackprof-gprof2dot wrappers