add fromString method for library consistency

lib/bn.js

@@ -18,6 +18,12 @@ var reversebuf = function(buf, nbuf) {
   }
 };
 
+bnjs.prototype.fromString = function(str) {
+  var bn = bnjs(str);
+  bn.copy(this);
+  return this;
+};
+
 bnjs.fromBuffer = function(buf, opts) {
   if (typeof opts !== 'undefined' && opts.endian === 'little') {
     var nbuf = new Buffer(buf.length);

test/bn.js

@@ -66,7 +66,23 @@ describe('BN', function() {
 
   });
 
-  describe('#fromBuffer', function() {
+  describe('#fromString', function() {
+    
+    it('should make BN from a string', function() {
+      BN().fromString('5').toString().should.equal('5');
+    });
+
+  });
+
+  describe('#toString', function() {
+    
+    it('should make a string', function() {
+      BN(5).toString().should.equal('5');
+    });
+
+  });
+
+  describe('@fromBuffer', function() {
     
     it('should work with big endian', function() {
       var bn = BN.fromBuffer(new Buffer('0001', 'hex'), {endian: 'big'});
@@ -83,6 +99,10 @@ describe('BN', function() {
       bn.toString().should.equal('1');
     });
 
+  });
+
+  describe('#fromBuffer', function() {
+
     it('should work as a prototype method', function() {
       var bn = BN().fromBuffer(new Buffer('0100', 'hex'), {size: 2, endian: 'little'});
       bn.toString().should.equal('1');