refactor(pkix): switch to crypto/x509's CertificateRequests

The `crypto/x509` package now supports CertificateRequests as of Go 1.3. However, not all operating systems have Go 1.3 yet in their repositories, which may make this PR undesirable. 

As of Go 1.3, PKCS #10 CSR support is present in the Go standard library. This change removes the third-party pkcs10 dependency, and switches the pkix CSR handling code to use the standard library.

The checkSignature function added to csr.go comes from CFSSL (https://github.com/cloudflare/cfssl), which is covered under the 3-clause BSD license.