js: improve decryption performance

• src/Connection.js, src/Helpers.js: performance of decoding incoming base64 data was improved to about 4 times the speed by introducing two more efficient functions (thanks to rrenkert@intevation.de for finding and testing them)

• src/gpgmejs.js: Decrypted data will now return as Uint8Array, if the caller does not wish for a decoding. Decoding binary data will return invalid data, and a Uint8Array may be desired. This can be indicated by using the (new) 'binary' option in decrypt.

• src/Errors.js A new error in case this decoding fails

• src/Message.js, src/Connection.js: expected is change from base64 to binary, to avoid confusion later on.