Added unusedbits parameter to ASN1BitString to specify number of padded bits.

This allows to manually create ASN1BitStrings for any string of bits (regardless of length).
Closes #5
Added isEncoded bool to ASN1Object
Added explicit BigInteger getters to ASN1Integer

This is useful so that users don't need to perform the type check on intValue. Also the positive value getter is also present in BounceCastle's ASN1 implementation and I needed it in Dartcoin.
Added a test to check encoding against DER specification

This test checks the encoding of several ASN1 primitives to the encoding in DER. All existing types that I could find a test case for (Integer, BitString and OctetString) pass the test. I'm pretty sure ASN1Length is also DER-compliant, so that would mean Sequence as well. Which means basically everything is.

I also created a test in Dartcoin to test for signature encoding to DER and it passed using asn1lib.