v5.0.0

• travis: update node versions (#205)
• Refactor buffer constructor (#200)
• lib: fix for negative numbers: imuln, modrn, idivn (#185)
• bn: fix Red#imod (#178)
• check unexpected high bits for invalid characters (#173)
• document support very large integers (#158)
• only define toBuffer if Buffer is defined (#172)
• lib: better validation of string input (#151)
• tests: reject decimal input in constructor (#91)
• bn: make .strip() an internal method (#105)
• lib: deprecate .modn() introduce .modrn() (#112 #129 #130)
• bn: don't accept invalid characters (#141)
• package: use files insteadof .npmignore (#152)
• bn: improve allocation speed for buffers (#167)
• toJSON to default to interoperable hex (length % 2) (#164)