add new option :sigalg

`jarsigner` causes error when not matched private key's algorithm (`-keystore`'s key's algorithm) and sign's algorithm (`-sigalg`) on http://developer.android.com/tools/publishing/app-signing.html#signapp .
And, lein-droid use `-sigalg` to `MD5withRSA` now. It requires `RSA` private key.
But, I generated `DSA` private key in past times and used it for past Android apps.
So, I want to change `-sigalg` to `SHA1withDSA` for support `DSA` private key.

I add `:sigalg` new option for it.
How does this look?